Liquid crystal display devices in TN (Twist Nematic) mode have well-balanced characteristics needed for display devices. For example, the devices have a low driving voltage and a relatively fast response speed, and are suitably used for color display because the devices provide monochrome display in principle. Therefore, such liquid crystal display devices in TN mode have been widely used for matrix-type liquid crystal display devices such as an active matrix-type liquid crystal display device and a simple matrix-type liquid crystal display device. However, such devices in TN mode also have disadvantages, such as a narrow viewing angle and a low contrast ratio.
Liquid crystal display devices in VA (Vertical Alignment) mode, which has a high contrast ratio, have been recently developed. In VA mode, liquid crystal molecules align substantially vertically to substrates when no voltage is applied between the substrates, and on the other hand, the liquid crystal molecules align substantially parallel to the substrates when a voltage sufficiently greater than the threshold voltage is applied between the substrates. Domain division techniques of dividing alignment directions of liquid crystal molecules in one pixel region have been developed. These techniques enable one pixel region to have a plurality of regions where the alignment directions of the liquid crystal molecules are different (herein after, also referred to as “domain”). As a result, the liquid crystal display devices can provide a wider viewing angle.
In addition, the following liquid crystal display devices in VA mode in which domain division is provided have been practically used. Liquid crystal display devices in MVA (Multi-Domain Vertical Alignment) mode in which, as an alignment control structure, one substrate is provided with electrode slits and the other substrate is provided with projective structures to perform the domain division; and liquid crystal display devices in PVA (Patterned Vertical Alignment) mode in which, as an alignment control structure, both substrates are provided with electrode slits to perform the domain division. These modes can provide liquid crystal display devices having a high contrast ratio, which is an advantage in VA mode, and a wide viewing angle, which is an advantage in the domain division.
However, the liquid crystal display devices in MVA and PVA modes have room for improvement in slow response speed. That is, only liquid crystal molecules near the electrode slits and the projective structures fast start to respond, even if a high voltage is applied to change black state to white state, and liquid crystal molecules, which are far from the alignment control structures, respond late.
For improvement in this response speed, it is effective that alignment treatment is provided for alignment films formed on the liquid crystal layer side surfaces of substrates, whereby to provide liquid crystal molecules with a pretilt angle previously. Also in VA mode, the liquid crystal molecules are previously made slightly incline toward the vertical alignment films, and thereby the liquid crystal molecules can easily incline when a voltage is applied to the liquid crystal layer. As a result, the response speed can be faster. As a method of the alignment treatment for providing the liquid crystal molecules with the pretilt angle, rubbing method, SiOx oblique deposition method, and photo-alignment method may be mentioned, for example.
The domain division is performed to obtain a wide viewing angle in MVA mode and PVA mode. However, there is room for improvement in that the number of the alignment treatment process for the alignment films increases if the domain division is performed. In the photo-alignment method, a domain division method of performing exposure through a photomask more than one time has been proposed, for example. It is preferable in terms of simplification of the production processes that the alignment treatment is performed with a small number of times. However, one pixel region has preferably two or more domains, and most preferably four or more domains in order to secure a wide viewing angle. Therefore, a method which can secure many domains with a small number of alignment treatments has been desired.
As the VA mode in which domain division is provided, a VA mode using vertical alignment films in which alignment directions on each other's substrates are antiparallel in any domain, as shown in FIGS. 8(a) and 8(b), (herein after, also referred to as VAECB (Vertical Alignment Electrically Controlled Birefringence) mode) has been proposed. In VAECB mode, as shown in FIG. 8(a), the direction of the absorption axis of first polarizer 35 formed on the first substrate side and the direction of the absorption axis of second polarizer 36 formed on the second substrate side are out of alignment with the alignment direction 31a of the first alignment film 31 and the alignment direction 32b of the second alignment film 32 by 45 degrees. In a mode of dividing one pixel region into four domains, which is particularly excellent in viewing angle (herein after, also referred to as 4VAECB mode) in VAECB mode, throughput in volume production decreases since the alignment treatment is performed in four directions, i.e. 45, 135, 225, and 315 degrees when the horizontal direction (azimuthal angle) on the display surface is defined as 0 degree, as shown in FIG. 8(b). For example, Japanese Kokai Publication No. 2001-281669 discloses a technique of performing alignment treatment by a photo-alignment method and thereby providing the VAECB mode. However, in this case, the exposure is performed for the alignment films a total of eight times.
In contrast, VAHAN (Vertical Alignment Hybrid-aligned Nematic) mode, in which one substrate is provided with a vertical alignment film subjected to no alignment treatment, can decrease the number of the alignment treatment. However, there is room for improvement in the response speed since the pretilt angle of the liquid crystal molecules remains 90 degrees on the other substrate side.
With this problem, a VA mode using vertical alignment films in which alignment treatment directions on each other's substrates are perpendicular to cause liquid crystal molecules to form a twist structure (herein after, also referred to as VATN (Vertical Alignment Twisted Nematic) mode) has been proposed (for example, with reference to Japanese Kokai Publication No. Hei-11-352486, Japanese Kokai Publication No. 2002-277877, Japanese Kokai Publication No. Hei-11-133429, and Japanese Kokai Publication No. Hei-10-123576). In a liquid crystal display device in VATN mode, as shown in FIG. 5(a), the first alignment film 31 and the second alignment film 32 align the liquid crystal molecules 33 with negative dielectric anisotropy substantially vertically to the alignment film surfaces, and align the liquid crystal molecules 33 near the first alignment film 31 and the liquid crystal molecules 33 near the second alignment film 32 such that the alignment directions of them are perpendicular to each other, when no voltage is applied between the substrates interposing the liquid crystal layer (at OFF-state). Each of the liquid crystal molecules 33 near the surfaces of the first alignment film 31 and the second alignment film 32 has the pretilt angle 34 to the alignment films. As shown in FIG. 5(b), the liquid crystal molecules 33 align in the direction parallel to the substrate surfaces as a voltage is applied between the substrates interposing the liquid crystal layer, depending on the applied voltage, and show birefringence to light transmitted through the liquid crystal layer. In VATN mode, as shown in FIG. 6(a), the direction of the absorption axis of first polarizer 35 and the alignment direction 31a of the first alignment film 31 are the same, and the direction of the absorption axis of second polarizer 36 and the alignment direction 32b of the second alignment film 32 are the same. Alternatively, as shown in FIG. 6(b), the direction of the absorption axis of first polarizer 35 and the alignment direction 32b of the second alignment film 32 may be the same, and the direction of the absorption axis of second polarizer 36 and the alignment direction 31a of the first alignment film 31 may be the same. As shown in FIG. 7, a mode of dividing one pixel region into four domains in VATN mode (herein after, also referred to 4VATN mode) needs only four times of the alignment treatment, which is half the number of times in 4VAECB mode. Such VATN mode is theoretically dramatically excellent in that a wide viewing angle and a fast response speed can be provided with a small number of processes. However, a technique of producing the liquid crystal display device in VATN mode has not been established yet. Additionally, the liquid crystal display device in VATN mode is difficult to produce stably because variation in the pretilt angle has a large influence on the transmittance as compared with the liquid crystal display device in VAECB mode.
In a production process of a liquid crystal display panel, a panel substrate to be used becomes larger year by year in order to improve production efficiency and the like. With the enlargement of the substrate size, a large photomask has been needed in the photo-alignment method which performs the exposure through a photomask. However, use of the large photomask has room for improvement in that distortion and the like is generated in the photomask and thereby accuracy of the exposure is reduced. A photomask having high-definition openings is extremely expensive, and therefore, there is room for improvement in that use of the large photomask increases manufacturing costs. With this problem, a method of moving a light source or a substrate (herein after, also referred to as “scanning exposure”) has been proposed as an exposure method (for example, with reference to Japanese Kokai Publication No. Hei-09-211465, and Japanese Kokai Publication No. Hei-11-316379).
However, there is still room for improvement in that domains having small variation in the characteristics are formed with a small number of alignment treatments if domain division is performed by a photo-alignment method.